COUP-TFs (Chicken Ovalbumin Upstream Promoter-Transcription Factors) are members of the Steroid/Thyroid Hormone Receptor superfamily. Since their ligand is not known, they are classified as orphan receptors. Molecular biology studies indicate that they are negative regulators which can inhibit the basal promoter activity of many genes as well as interfere with many transcription activators such as thyroid hormone, retinoic acid, vitamin D. estrogen. retinoid X, and peroxisome proliferator activated receptors. To dissect the physiological roles of the COUP-TFs, we have carried out expression studies of the COUP-TFs during mouse embryonic development. These studies support the hypothesis that COUP-Tfs are important for the cell type specific differentiation in central nervous system and they also play a primordial role in epithelial-mesenchymal interactions during organogenesis of lung, kidney, prostate, skin, salivary gland, pancreases and gastrointestinal organs. In this application, we propose experiments to verity these hypotheses. Specifically, we will use homologous recombination to knock-out the COUP- TFI and II genes either in the whole animal or in a few specific tissues to study their role(s) during development and differentiation of the central nervous system as well as various organs. In addition, we will use an in vitro reconstitution system between mesenchymal and epithelial cells to confirm that COUP-TFs indeed are involved in regulating the paracrine signals necessary for the mesenchymal epithelial interactions. Finally, we will also determine the signals regulating the COUP-TF expression during embryonic development. These studies should help us to realize our long-term goals in understanding the signaling pathways which require the involvement of the COUP-TF gene activities eventually resulting in the final cellular differentiation of various tissues. It is expected that the understanding derived from this project will be relevant to the biology of development and differentiation. The proposed studies should also be pertinent to development of more precise theories for the biochemical mechanism of action of hormones in general and more specifically thyroid, retinoic acid and vitamin D.